how much will it cost to fix the mic not working on an iphone 11

Hey there Joseph,

While we're unaware of which steps you tried to determine service is required, we recommend first trying the steps here as they address the issue you described: If the microphones on your iPhone aren't working

Under the section 'How much will it cost?', it has an estimate tool for service: iPhone Service & Repair

If you need more help with setting up service, or even reviewing your service options, we recommend contacting Apple Support as they can help with answering those questions: Get Support

Cheers!